The specific aims of the project are to: 1) characterize molecular differences that distinguish avirulent Acanthamoeba; 2) investigate the mechanism of encystment induction in Acanthamoeba; 3) determine the effectiveness of 5-fluorocytosine (5-FC) as a chemotherapeutic in experimental mouse amebic encephalitis; 4) elucidate the biochemical mechanism of action of 5-FC as an amebostatic or amebocidal agent. The current studies have involved biochemical analysis on isolated plasma membranes in addition to comparative analyses of lectin-induced agglutination of pathogenic and non-pathogenic strains. Additional research to characterize virulent strain relies on determining the extent of genetic homology between pathogenic and non-pathogenic strains. Results from in vivo and in vitro testing of 5-FC indicate that the clinical usefulness of this drug may be limited by the amebas' capacity to develop resistance to 5-FC. Studies that represent a new direction of the project have been initiated on Naegleria strains, which have been isolated from human cases of primary amebic meningoencephalitis. The investigations being emphasized include biochemical aspects of the cytopathogenicity of Naegleria in human tissue cultures along with a comparative analysis of lectin-induced agglutination of the various isolates.